Ultrafast Carrier Recombination and Transient Lattice Temperature Changes in 25 nm Thin Hydrogenated Amorphous Silicon Films

Hydrogenated amorphous silicon (a-Si:H) represents an important material for optoelectronic and photovoltaic applications. Fundamental knowledge regarding its charge carrier dynamics and thermalization behavior after photoexcitation is therefore required. Here, we provide a detailed broadband UV–vis–NIR transient absorption study of 25 nm thin a-Si:H films on glass upon pumping at 390, 400, or 650 nm, covering the wavelength range 300–1600 nm. Photoexcited carriers relax in the extended states within a few hundred femtoseconds and recombine very quickly. In contrast to a previously suggested bimolecular recombination mechanism for carrier densities larger than 5 × 1018 cm–3, we unambiguously identify Auger-type third-order recombination with the rate constant k3 = (2.3 ± 0.6) × 10–29 cm6 s–1, which is extracted from the absorption decay in the NIR region. The released recombination energy leads to a fast lattice heating which is indicated by a fast rise of absorption in the UV–vis region. This spectral feature is due to a transient red-shift of the a-Si:H absorption spectrum, i.e., a transient reduction of the Tauc band gap. On time scales larger than 10 ps, the lattice temperature gradually decreases by heat transfer to the glass substrate. The change of the lattice temperature is extracted quantitatively from the transient spectra. Complementary experiments using time-resolved step-scan FTIR absorption spectroscopy characterize slower thermalization processes of a-Si:H on the microsecond time scale. Measurements of the external quantum efficiency of these thin a-Si:H layers between 360 and 1000 nm using wavelength-resolved photocurrent experiments and step-scan Fourier-transform photocurrent spectroscopy are provided.